In a fibre optical communications system, it is desirable to be able to control the modulation depth of the light generated by the transmitting laser device. In order to maintain fast switching between states and reduce noise, the transmitting laser is not switched to some defined power and then switched off, but for the low state its output is reduced to a low level. This modulation depth is also described as an extinction ratio (ER), the latter being the ratio of the optical intensity when there is a data ‘1’ and the intensity when there is a data ‘0’. The current required by the laser to deliver these high and low optical outputs is not however constant and indeed is affected by the tolerances between individual laser devices, and also over time, due to the variation of a single laser device's characteristics due to heating and ageing. Such variations can occur in normal operation as a device heats up in use.
Hence it is desired not only to be able to compensate automatically for manufacturing tolerances and parameter drift in the laser itself, but also to be able to reach some defined target modulation level where the data pattern has a random characteristic with only limited low frequency content.
One method of providing a closed loop control for the laser output is to create a replica of the monitor photodiode output current by taking the incoming modulation data stream and using this to control reference currents and passing the combined current to a replica transimpedance amplifier (TIA) and associated circuitry. After suitable signal processing of the monitor and the replica path signals, the two may be compared in some manner and error information generated to facilitate closed loop control of the average optical power and the extinction ratio.
In order to construct such a closed loop control system using a replica path, it is necessary to be able to create a current signal that is equivalent to the current from an ideal photodiode which sensing the optical power output from the transmitting laser. This current signal must have not only the same average value as the ideal photocurrent, but also possess the same modulation depth or extinction ratio (ER). Hence it is necessary to be able to create currents that may be switched and combined so as to provide said replica of an ideal photocurrent with any desired value of ER. Analysis shows that the current values needed for this do not have a linear relationship with any convenient input variable. In most cases, a user might wish to set the target ER level in terms of decibels, and to be able to vary said ER in steps of decibels.
It is an object of some of the embodiments to provide currents suitable for such a control system where the relationship between the variable representing the demanded ER and the currents is achieved in a particularly convenient way and with attention to the minimisation of errors in creating said currents.